Support signing assemblies in the C# backend.

Add a new option, `--sign-assembly', to the Mercury compiler that causes it to
sign library assemblies generated by the C# backend with a user-specified
strong name.  There was an existing undocumented option of the same name for
use by the IL backend; it has been renamed to --il-sign-assembly.

compiler/options.m:
	Add the new option.

	Rename the existing `--sign-assembly' option to `--il-sign-assembly'.

compiler/compile_target_code.m:
	Optionally sign library assemblies.

compiler/mlds_to_il.m:
compiler/mlds_to_managed.m:
compiler/write_deps_file.m:
	Conform to the above changes.

doc/user_guide.texi:
	Document the new option

NEWS:
	Announce the new option.